The description below relates in particular to the front lighting of a display device such as a reflective liquid crystal display cell. This technique is known as “front light illumination”.
A known technique for the front light illumination of a reflective type display device consists in arranging a transparent light guide, inside which light is injected, above the display device. The light guide is structured to allow gradual extraction of the light that is propagating in the direction of the underlying display device. The structures provided in the light guide for extracting the light are called “light extractors”. The number, geometry and distribution of these extractors inside the light guide vary in accordance with the application thereof.
It is an arduous task to select a given type of light extractor which can provide a good level of illumination without adversely affecting the display contrast of the data provided by the display device. Indeed, for mainly aesthetic reasons which are easily understood, these light extractors must, in particular, be as little visible as possible to the user and able to ensure the most uniform possible lighting over the entire surface of the display device. This latter condition is especially important for the illumination of a display device that is integrated in a wristwatch.
The first problem which arises with the front light illumination of a reflective display device by means of a light guide and which adversely affects illumination uniformity is linked to the actual operating principle of a light guide of this type.
Where the luminous flux or luminous power injected by a light source at the light guide inlet is called P0 and assuming that the guide has a light extraction efficiency per unit of length E, the internal luminous flux remaining inside the light guide according to the distance z travelled by the light inside the guide is given by the formula P0e−E.z. Consequently, the luminous flux Pextr(z) that can be extracted from the guide at a given point z on the length of the guide is proportional to the internal luminous flux in accordance with the relation:Pextr(z)∝P0e−E.z  (1)
The above equation (1) shows that if the light extraction efficiency E remains the same over the entire length of the light guide, the luminous flux Pextr(z) extracted from the guide decreases exponentially with the distance from the point of injection of light into the guide. Consequently, the illumination of the display device will not be uniform over the entire surface thereof.
To overcome this problem, a first solution might consist in creating a light guide whose light extraction efficiency varies with the distance from the point of injection of light into the guide. More specifically, the light extraction efficiency of the guide would have to increase progressively away from the point of injection of light into the guide. This solution is, however, difficult to implement. By definition, the light extraction efficiency always remains less than 1. Consequently, the extraction efficiency cannot increase indefinitely with the distance from the point of injection of light into the guide. Further, to obtain a continuous variation in light extraction efficiency along the guide, the geometry or depth of the light extractors in the guide would have to be continuously altered, which would require considerable effort in terms of the design and fabrication of the light guide.
Another solution for compensating for the exponential decrease in the luminous flux extracted from the light guide could consist in providing a guide having constant and very low (i.e. much lower than 1) light extraction efficiency along the entire length of the guide. In such case, equation (1) above could be approximated as follows:Pextr(z)∝P0e−E·z≈P0(1−E·z)≈P0  (2)
Equation (2) above shows that with constant and very low light extraction efficiency, the luminous flux extracted from the light guide is substantially the same over the entire length of the guide and is equal to the luminous flux injected into the guide inlet. In practice, this solution is much easier to achieve since it does not require varying the geometry of the extractors along the guide. Indeed, the light extraction efficiency must the same over the entire length of the optical guide. However, this second solution also raises a problem. Indeed, to be verified, equation (2) above requires the light extraction efficiency to be very low. In other words, only a small part of the luminous flux initially injected into the light guide is really used for lighting the reflective display device. The rest of the luminous flux injected into the light guide will be expelled or absorbed once it reaches the outlet of the guide and therefore lost forever. Consequently, the optical output ratio of this type of system is mediocre.
To overcome this problem, one could envisage arranging a mirror at the light guide outlet so as to re-inject into the guide any light that escapes therefrom. This solution is not however very appropriate for the type of light guide with which we are concerned here. Indeed, once reflected back inside the guide, the light propagates in an opposite direction to the initial direction of propagation in the guide and no longer interacts with the extractors in the same way, since the latter generally have an asymmetrical structure which is optimised for extracting light originating directly from the source arranged at the guide inlet. Any light propagating in the opposite direction will not therefore be extracted or diffused in an optimum manner and will adversely affect the display contrast of the display device.
It is an object of the present invention to overcome the aforementioned drawbacks in addition to others by providing a light guide having improved an optical output ratio.